Welcome to the community. There are a couple of common reasons a toilet might continually run. Luckily both are simple enough solutions.

The fill valve might be set to allow to much water into the toilet tank. This will cause the excess water to flow into the overflow tube.

Depending on which fill valve assembly you have the water level will need to be adjusted lower. On a traditional ballcock (float suspended at the end of an arm) the screw at the top of the flush tower will need to be loosened. The screw is noted in the image below. On more recent toilets the remedy is the same only the location is slightly different. The adjustment screw is listed in the second image.

Flappers that have warped or deteriorated with time can also can also allow water to slip by causing the toilet to run. The solution is to replace the flapper. Flappers can vary greatly by manufacturer so I always suggest to customers to bring in there flapper for an accurate match.

In some cases however even these solutions may not be enough. In those cases I recommend replacing the fill valve completely. If the valve is original to the toilet and somewhat old replace the the flush valve as well. Fluidmaster provides a complete repair kit to accomplish this.